You know, I didn't think there was anything there based on the data you posted, but I fit a model and it actually looks like time is quite significant.
The obvious thing to do is just fit a line to the data, which is what I did. I don't know of any other options, without knowing more about the process involved. If you wait 6 months, I'll have taken a course on longitudinal data and then I'll have a better answer.
Warning: You should only be testing this if you were expecting to find a trend in time BEFORE you did the experiment. It is very bad form to test hypotheses suggested by the data on the same data set.